How can I tell if a sponge is aquarium safe?

not the biggest fan of sponges at all, due to thier chem/bio harboring capabilities. Depending on what the problem is, use something else.

an expired credit card works great for scraping algae off the glass

so would a non expired card lol

I use one of the single sided razor blades to scrape off the hard water and salt deposits. Never thought about trying to use it on algae. Well... never thought about using a CC for it either. lol I know they work for a frosty car window though.

Any sponge that does not specify it is safe most likely will not be good to use. Any sponges that contain any sort of detergent, or lists something about preventing mildew is not safe, and potentially lethal.

Just my 2 cents, but my tanks are not worth risking on a sponge. Spend the money to get ones you KNOW are safe. For me, a cost of $10/sponge would be better tahn killing 5 $10 fish, and you can get safe sponges from any LFS much cheaper.

I concur with Oriongirl....most sponges have anti-mildewants impregnated into the sponge material during formation, and most do not state so anywhere on the packaging...in fact, often only the guys on the manufacture floor can tell you for sure. Too much to risk. If you must know, put it in a bucket of water for a couple of hours, then add some feeder guppies, and see how many are alive come morning, but it is a waste of good guppies.

I agree, I would never use anything for the inside of an aquarium that was specifically specified for it. That's one thing I wouldn't risk, even papertowels in my experience have something in them not safe. I don't know if all of them do but just that one was cause enough for my concern.